The present invention relates generally to the field of pipeline pigs and, more particularly, to a molded pipeline pig having various components of different hardnesses or different configurations molded together in two or more molding steps. While the pig of this invention may be used for onshore pipelines, it was specially conceived for offshore oil pipeline applications wherein a non-metallic molded structure is mandated.
The buildup of coatings such as paraffin on the interior of surface of a pipeline is a common problem in the oil industry. Paraffin buildup on the interior surface of the pipe restricts the flow capacity of the pipeline. Today, pipeline operators typically use a pig to remove such undesirable coatings from the interior of pipelines. A pig commonly comprises a metal body or mandrel that supports one or more flexible scraping disks, sealing disks, flower petal disks, and/or cups. The disks are ordinarily made of some type of rubber, polyurethane, or thin metal. These kinds of pigs work well in many applications, but unfortunately, if the metal mandrel pig suffers a catastrophic failure or comes apart while inside the pipeline, the metal fragments from the mandrel may become lodged in the wall of the pipeline or in valves or pipeline junctions, or they may damage downstream equipment such as pumps or sensors. Since such pigs are commonly bolted together from a number of individual parts, the bolts may come undone or fail, and for these and other reasons many critical applications require the use of non-metal pigs.
Pigs having a metal mandrel also have difficulty in traversing sharp bends in the pipeline. In sharp bends in the pipe, the rigid metal mandrel may prevent the pig from successfully navigating the bend. There are some metal mandrel pigs that have a universal joint in the mandrel that enables the pig to pass a bend. However, the universal joints add cost, and present another mechanism that is subject failure within the pipeline.
Completely non-metallic pigs eliminate these drawbacks. If a non-metallic pig breaks up inside the pipeline, the fragments of the pig ordinarily eventually degrade in the flowing fluid. Also, another pig may be sent through the pipeline to either destroy or dislodge the fragments. While the non-metallic pigs do not present the disadvantages associated with the metal mandrel pigs, they nevertheless present a further disadvantage that becomes readily apparent in pipelines of variable internal diameter.
To facilitate cleaning and maintenance, a pipeline should have a constant internal diameter. However, there are many circumstances where two sections of pipeline, each having a different internal diameter, are joined together, such as for example to form a xe2x80x9cYxe2x80x9d. In such circumstances, a pig having a given diameter maybe satisfactory to clean the interior of one of the pipeline sections, but not the other. For example, the cleaning disks and/or cups on the all-rubber pig may not be sufficiently flexible to enable the pig to readily move from a relatively larger diameter pipe section to a relatively smaller diameter pipe. Conversely, a pig that is effective in cleaning a smaller diameter pipe may not properly seal against the interior surface of the pipe as it passes into the larger diameter pipe. For pigs that do have sufficiently flexible cleaning disks, there is the further risk that, as the pig encounters a reduced internal diameter pipe section, and the disks are folded backward, buckling may occur. As the disks buckle, the peripheral surfaces of the disks will have a tendency to form folds and ripples. The buckling is a natural consequence of the overgauged disks being compressed into the undergauged internal diameter of the second section of pipe.
One solution to these problems was suggested by Minton in U.S. Pat. No. 5,457,841. In Minton, a non-metallic pipeline pig for cleaning a variable diameter pipeline comprises an elongated cylindrical body, front and back cups coupled to the body, and a plurality of generally circular wiping disks coupled to the body between the cups. Minton suggests that the disks and front and back cups could be molded as a single unit, or separately molded and later joined by a second molding process or by applying a suitable adhesive. How this is to be done is not at all clear from the description in Minton. The material from which the pig is formed is preferably polyurethane rubber with a durometer of between 60 and 80 inclusive. One drawback to the Minton pig, however, is that apparently no provision is made for one or more of the disks having one hardness, while one or more other disks have a different hardness. Thus, in using the Minton pig, for example, in a pipeline having a large section coupled to smaller section, the disks will bend back and therefore provide no scraping function, unless an abrasive is also used as disclosed in Minton.
Thus, there remains a need for a pipeline pig in which disks of different types, or disks of the same type but of different durometers, may be molded separately and then molded onto a polyurethane mandrel in a separate molding step. Further, there remains a need for a method of forming a pipeline pig which includes these features. The present invention is directed to just such a pipeline pig and to a method of making the pig.
The present invention solves these and other drawbacks in the art by providing a method of forming a pipeline pig in two or more steps. Selected disks and/or cups, of selected size and hardness, are first formed. These disks and/or cups may include scraper disks, seal disks, flower petal disks, and/or cups in various sizes and hardnesses, in any desired permutation of these characteristics. As used herein, the term xe2x80x9claterally extending pig componentxe2x80x9d or the term xe2x80x9cradially extending pig componentxe2x80x9d refers to these and other components of a pipeline pig adapted to contact the inner surface of a pipe. The preformed pig components are then arranged with a plurality of mold pieces placed between the various disks and/or cups and then a second molding step is performed.
The pig components which are formed in the first step preferably include a large central hole through which polyurethane will flow in the second molding step to form the mandrel. The pig components also preferably include a plurality of smaller holes radially spaced around the large central hole. The smaller holes serve to form a mechanically robust coupling between the pre-formed pig components and the polyurethane mandrel. Thus, the various pig components maybe combined in any desired permutation of characteristics, so that a pig with desired working properties may be tailored to a specific task.
These and other features and advantages of the present invention will be apparent to those skilled in the art from a review of the following detailed description along with the accompanying drawings.